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Old 11-05-2021, 06:52 PM   #1 (permalink)
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Increasing Air Intake Diameter

Had a thought and wasn't sure what the result would be.

If you increased the diameter of the air intake tube, all else equal what would the computer do to the a/f ratio? (Let's assume the length and the diameter at the MAF is the same.) Would the computer just compensate, or would the extra air in the tube mess up the preset calculations the computer has?

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Old 11-06-2021, 10:42 AM   #2 (permalink)
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Had a thought and wasn't sure what the result would be.

If you increased the diameter of the air intake tube, all else equal what would the computer do to the a/f ratio? (Let's assume the length and the diameter at the MAF is the same.) Would the computer just compensate, or would the extra air in the tube mess up the preset calculations the computer has?
Regarding the air/fuel ratio, nothing would happen. The o2 sensors would allow the ECU to compensate.

The larger diameter would slow down the air flow and could cause a loss in power.
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Old 11-06-2021, 01:10 PM   #3 (permalink)
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Regarding the air/fuel ratio, nothing would happen. The o2 sensors would allow the ECU to compensate.

The larger diameter would slow down the air flow and could cause a loss in power.
I agree. The MAF sensor is calibrated for a certain size intake diameter. If you change the diameter (to much) you will change the velocity of the air. The O2 sensors should keep the A/F ratio correct but it would do it by pulling fuel back because there’s more air than the MAF told the ECM.

I buddy just experienced this is an engine swap in a Jeep where he upsized from 3” to 4” with the stock MAF sensor.
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Old 11-06-2021, 09:22 PM   #4 (permalink)
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The larger diameter would slow down the air flow and could cause a loss in power.
Going the other direction... would decreasing the diameter speed up the air flow increasing power? (within reason of course, not downsizing to 1/2" or something)

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I agree. The MAF sensor is calibrated for a certain size intake diameter. If you change the diameter (to much) you will change the velocity of the air. The O2 sensors should keep the A/F ratio correct but it would do it by pulling fuel back because there’s more air than the MAF told the ECM.

I buddy just experienced this is an engine swap in a Jeep where he upsized from 3” to 4” with the stock MAF sensor.
What is too much? Is 1/4" within limits? Say 2-3/4" to 3" diameter pipe?

How does the Jeep run with the 1" larger pipe? Does it have more power, more "zip", better throttle response? Or just the opposite?
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Old 11-07-2021, 06:20 PM   #5 (permalink)
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Depends on a number of things. How is the air flow being measured? There are three different ways this typically happens (or four if you add carburetors to the discussion).

One is "just guess based on the throttle position". This is the Alpha-N strategy, and I don't think it's used by any modern automakers.

Another is to measure manifold air pressure. When combined with the knowledge of how large the manifold is, the amount of air the engine takes in during a full cycle, and other information about the intake system, this allows you to guess pretty reasonably about the amount of air going in. The earliest electronic FI (Bendix Electro-Jet, Bosch D-Jetronic) used this, and a number of Honda products at least through the mid-1990s did as well.

The last uses a device in the air stream to measure the flow. Combined with the temperature, this gives you a very good idea of the mass of air flowing in. Most of these air flow meters are a fixed size, so changing the diameter of the pipe that goes to them or that leads from them to the manifold won't do much to help air flow. In fact, it may harm air flow in some flow regimes due to the tube changing size.

In a decent modern system, the O2 sensor will compensate for inaccurate measurements of air intake. But it will generally lag behind what is actually occurring, because it is reporting after the air is combusted and also takes some time to react after that point. So it will be a little laggy, which can produce odd transitional behavior in some circumstances if bad enough.


The upshot is: With a larger pipe you may gain a little top end power depending on all of the details of your motor and intake and fuel system. You are likely to lose a similar amount of mid-range torque, but you might not notice it. That's assuming you don't have a smaller section of pipe to fit your air flow meter. (If you do, you may find power losses in many places throughout the RPM/throttle ranges, and likely not much power.)

Pumping losses are likely to be similar to unmodified, because at throttle openings that we generally care about the throttle valve itself is by far the largest restriction to intake air flow.

That said, these are all generalisms. There can be and are specific instances where just putting a larger pipe on will help with power, or with pumping losses, or both. Direct experimentation will provide the answer in any specific case.
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Old 11-07-2021, 07:44 PM   #6 (permalink)
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You contemplate a subsystem that extends from the intake snorkel to the valve seats.
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If you increased the diameter of the air intake tube, all else equal
Are you talking about before or after the MAF [sensor]? Plenums or runners or both? There acoustical waves reverberating in the intake.

The Hot VWs Mileage Motor used 'stock aftermarket' manifolds but had smaller intake valves.
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Old 11-08-2021, 09:52 AM   #7 (permalink)
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Originally Posted by jeff88 View Post
Going the other direction... would decreasing the diameter speed up the air flow increasing power? (within reason of course, not downsizing to 1/2" or something)



What is too much? Is 1/4" within limits? Say 2-3/4" to 3" diameter pipe?

How does the Jeep run with the 1" larger pipe? Does it have more power, more "zip", better throttle response? Or just the opposite?

The question needs more information for clarity. Is the question in regards to everything in front of the throttle body or are you building a customer intake manifold?

If it is in front of the throttle body (stock throttle body) you can expect nearly zero gains. If you install a larger throttle body then yes, you may see better WOT performance but may suffer some part throttle drivability.


Also, what is your goal? What is the reason for the question?
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Old 11-08-2021, 02:29 PM   #8 (permalink)
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Are you talking about before or after the MAF [sensor]? Plenums or runners or both? There acoustical waves reverberating in the intake.
For the sake of knowledge let's say both before and after MAF. I know for my 4Runner as an example, the MAF is down line from the air box, so the pipe could conceivably be different pre and post MAF. I'm thinking pre-TB so the plenum wouldn't change.

What does acoustical waves do for power/efficiency?

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Also, what is your goal? What is the reason for the question?
I have an idea for my air box and started researching and saw some people changing pipe size, leaving everything else stock. Made me wonder what size changes can do for power/efficiency.
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Old 11-08-2021, 03:37 PM   #9 (permalink)
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What does acoustical waves do for power/efficiency?
Good question. Are intake runners tuned for length? I know exhaust better -- 4 into 1 vs 4 into 2 into 1, and antireversionary, headers.
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Old 11-09-2021, 11:09 PM   #10 (permalink)
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If you want to learn about naturally aspirated intake manifold design, this forum is likely not where you will find robust, peer reviewed literature.

I personally think your off in the weeds, it would be a significant undertaking with very minimal benefits to modify an existing or create your own intake manifold.

Also, still not sure, are you talking about the tube between the air filter box and throttle body or are you considering the intake manifold itself, the part that bolts to the top of the engine?

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